CDL Air Brakes TestUsing Air BrakesMedium
A commercial driver is performing a pre-trip inspection. They pump the brake pedal several times to reduce air pressure. The spring brakes (parking/emergency brakes) should automatically engage when the air pressure drops to a specific range. What is this typical pressure range for automatic spring brake application?
- A80-100 psi
- B20-45 psi
- C45-60 psi
- D60-80 psi
Show answer & explanationAnswer & explanation
Correct answer: B. 20-45 psi
Spring brakes are designed to engage automatically when air pressure drops to a critically low level, typically between 20 and 45 psi, to prevent the vehicle from rolling if air pressure is lost.
Why the other options are wrong
- A. This range is well within normal operating pressure; brakes would not engage here.
- C. While 45 psi is the upper end of the typical range, 60 psi is where the low air warning activates.
- D. This is above the typical range for automatic spring brake application; the low air warning should activate here.
Spring Brake Application Pressure
The air pressure range at which the spring brakes (parking/emergency brakes) in an air brake system will automatically engage due to insufficient air pressure, typically between 20-45 psi.
- Activates automatically when air pressure is too low.
- Provides emergency braking if service air fails.
- Also used for parking the vehicle.
Memory trick: Spring Brakes 'Snap' from Twenty to Forty-Five!